Millimeter-wave communication technology refers to a technology using an electromagnetic wave whose wavelength is one centimeter to one millimeter (corresponding to the frequency range of 30 GHz to 300 GHz) to communicate. Currently, civil millimeter-wave communication technology mainly uses the spectrum whose frequency band is about 60 GHz, which is mainly derived from the following factors: firstly, many countries and regions plan to use the spectrum close to the 60 GHz as permission-free frequency band, which has the biggest advantage of free use; secondly, in the most recent period, as the technology becomes more and more mature, the performance and power consumption index of the 60 GHz millimeter-wave device have been enhanced greatly and have basically met the practical needs; thirdly, the 60 GHz wireless communication technology gradually develops from academic theory research to practical application research in industry, currently there already are several international standard organizations completed or mostly completed the 60 GHz international standards, such as IEEE 802.11ad and IEEE 802.15.3c as well as European ECMA-387, with the active promotion of various chip manufacturers and communication companies. The IEC also released the International Standard 13156 based on the first release of ECMA-387 specification.
The biggest advantage of 60 GHz technology is very wide transmission bandwidth and in the vicinity of 60 GHz, can provide up to 5 GHz transmission bandwidth, has a transmission rate of up to 1 Gps, and is that the occupied working frequency can be used without authorization. Because the electromagnetic spectrum is a strong absorption peak close to the 60 GHz, the electromagnetic wave propagation decay in this frequency range is very large, therefore the typical transmission distance of the 60 GHz communication technology is no more than 10 meters. This electromagnetic propagation property not only defines that the application scenario of the 60 GHz communication technology is primarily the indoor environment, but also makes the spatial multiplexing become possible.
The 60 GHz technology still has some technical difficulties, mainly originated from the 60 GHz electromagnetic wave propagation characteristics and the spatial channel characteristics determined thereby. For example: in the 60 GHz frequency band, the electromagnetic wave propagation is behaved more similar to light propagation, i.e., the 60 GHz antenna is primarily a directional antenna, using such an antenna cannot find the target device if the target device does not fall within the pointing range of the current device antenna pattern, thus generating the so-called “shadow phenomenon”. To solve this problem, in addition to adjusting the antenna directivity, it also designs the MAC layer specifically for the directional antenna, which will affect the efficiency of the MAC layer and reduce the valid data transmission to some extent.
The 60 GHz antenna may have features of narrow beam and directivity through the beamforming technology, so that in theory, the 60 GHz millimeter-wave communication technology can take advantage of three-dimensional space to perform transmission multiplexing so as to greatly increase the system capacity, and as shown in FIG. 1. But on the other hand, due to the features of the directional antenna and the presence of phenomena such as shadow and occlusion, the spatial multiplexing transmission is relatively difficult to implement, especially in the beamforming phase, which will cause relatively large interference. The beamforming can be roughly divided into two phases, the first phase: using the quasi-omnidirectional antenna to perform beam scanning, the second phase: after obtaining the general direction of transmission object after completing the beam scanning phase, performing a beam refinement. Wherein the first phase is mandatory, and the second phase is optional, but taking into account that the beam direction obtained in the first phase is relatively broad, the transmitting end and receiving end cannot be precisely aligned, therefore the second phase is generally required.
As shown in FIG. 2, if the device C and the device D need to perform a high-speed communication, they need to perform a beamforming operation. However, when the devices A and B are already in the high-speed communication state, the beam scanning operation desired by the beamforming conducted by the devices C and D may interfere with the current communication states of the devices A and B. Although the devices C and D can also choose other channels (channels other than the channels occupied by the links of the devices A and B) for transmission, such an operation is actually the spatial multiplexing concept that uses different frequencies instead of the same frequency for transmission.
In summary, currently, in the 60 GHz millimeter-wave communication, the spatial multiplexing transmission cannot be implemented without affecting the existing links.